think about it, you are bolting 9" of cast ally onto your driveshafts each side. Now you also have a 9"wide tyre in contact with the road. In order to move you need friction. The amount of friction you are going to get from them is going to cause them require more power to get them moving and keep them moving. Hence killing your power output and making you travel slower.
Archwork, yes, as i said before anything larger than a 7.5" wide wheel is more than likely to require arch work. especially at the fronts. As it needs to be able to turn within the arch without fouling either your driveshaft or your arch. At the rears if you fo for 9s then definately you will need to remove the lip and get it rolled. Removing the lip will require the arches being seam welded back up as they are made of 3 skins. If you don't do this the inside of the arch will rot away.
as blue mentioned, for looks yeah great, they look fat, if you want to keep a good speed level up then smaller alloys the better.
